Re: [PATCH 10/22] x86/traps: Move subarch_percpu_traps_init() into traps-setup.c

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 08/08/2025 9:23 pm, Andrew Cooper wrote:
> ... along with the supporting functions.  Switch to Xen coding style, and make
> static as there are no external callers.
>
> Rename to legacy_syscall_init() as a more accurate name.
>
> No functional change.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
>  xen/arch/x86/include/asm/system.h |  2 -
>  xen/arch/x86/traps-setup.c        | 97 ++++++++++++++++++++++++++++++-
>  xen/arch/x86/x86_64/traps.c       | 92 -----------------------------
>  3 files changed, 95 insertions(+), 96 deletions(-)
>
> diff --git a/xen/arch/x86/include/asm/system.h 
> b/xen/arch/x86/include/asm/system.h
> index 3cdc56e4ba6d..6c2800d8158d 100644
> --- a/xen/arch/x86/include/asm/system.h
> +++ b/xen/arch/x86/include/asm/system.h
> @@ -256,6 +256,4 @@ static inline int local_irq_is_enabled(void)
>  #define BROKEN_ACPI_Sx          0x0001
>  #define BROKEN_INIT_AFTER_S1    0x0002
>  
> -void subarch_percpu_traps_init(void);
> -
>  #endif
> diff --git a/xen/arch/x86/traps-setup.c b/xen/arch/x86/traps-setup.c
> index 13b8fcf0ba51..fbae7072c292 100644
> --- a/xen/arch/x86/traps-setup.c
> +++ b/xen/arch/x86/traps-setup.c
> @@ -2,13 +2,15 @@
>  /*
>   * Configuration of event handling for all CPUs.
>   */
> +#include <xen/domain_page.h>
>  #include <xen/init.h>
>  #include <xen/param.h>
>  
> +#include <asm/endbr.h>
>  #include <asm/idt.h>
>  #include <asm/msr.h>
>  #include <asm/shstk.h>
> -#include <asm/system.h>
> +#include <asm/stubs.h>
>  #include <asm/traps.h>
>  
>  DEFINE_PER_CPU_READ_MOSTLY(idt_entry_t *, idt);
> @@ -19,6 +21,8 @@ static bool __initdata opt_ler;
>  boolean_param("ler", opt_ler);
>  
>  void nocall entry_PF(void);
> +void nocall lstar_enter(void);
> +void nocall cstar_enter(void);
>  
>  /*
>   * Sets up system tables and descriptors for IDT devliery.
> @@ -138,6 +142,95 @@ static void load_system_tables(void)
>      BUG_ON(stack_bottom & 15);
>  }
>  
> +static unsigned int write_stub_trampoline(
> +    unsigned char *stub, unsigned long stub_va,
> +    unsigned long stack_bottom, unsigned long target_va)
> +{
> +    unsigned char *p = stub;
> +
> +    if ( cpu_has_xen_ibt )
> +    {
> +        place_endbr64(p);
> +        p += 4;
> +    }
> +
> +    /* Store guest %rax into %ss slot */
> +    /* movabsq %rax, stack_bottom - 8 */
> +    *p++ = 0x48;
> +    *p++ = 0xa3;
> +    *(uint64_t *)p = stack_bottom - 8;
> +    p += 8;
> +
> +    /* Store guest %rsp in %rax */
> +    /* movq %rsp, %rax */
> +    *p++ = 0x48;
> +    *p++ = 0x89;
> +    *p++ = 0xe0;
> +
> +    /* Switch to Xen stack */
> +    /* movabsq $stack_bottom - 8, %rsp */
> +    *p++ = 0x48;
> +    *p++ = 0xbc;
> +    *(uint64_t *)p = stack_bottom - 8;
> +    p += 8;
> +
> +    /* jmp target_va */
> +    *p++ = 0xe9;
> +    *(int32_t *)p = target_va - (stub_va + (p - stub) + 4);
> +    p += 4;
> +
> +    /* Round up to a multiple of 16 bytes. */
> +    return ROUNDUP(p - stub, 16);
> +}
> +
> +static void legacy_syscall_init(void)
> +{
> +    unsigned long stack_bottom = get_stack_bottom();
> +    unsigned long stub_va = this_cpu(stubs.addr);
> +    unsigned char *stub_page;
> +    unsigned int offset;
> +
> +    /* No PV guests?  No need to set up SYSCALL/SYSENTER infrastructure. */
> +    if ( !IS_ENABLED(CONFIG_PV) )
> +        return;
> +
> +    stub_page = map_domain_page(_mfn(this_cpu(stubs.mfn)));
> +
> +    /*
> +     * Trampoline for SYSCALL entry from 64-bit mode.  The VT-x HVM vcpu
> +     * context switch logic relies on the SYSCALL trampoline being at the
> +     * start of the stubs.
> +     */
> +    wrmsrl(MSR_LSTAR, stub_va);
> +    offset = write_stub_trampoline(stub_page + (stub_va & ~PAGE_MASK),
> +                                   stub_va, stack_bottom,
> +                                   (unsigned long)lstar_enter);
> +    stub_va += offset;
> +
> +    if ( cpu_has_sep )
> +    {
> +        /* SYSENTER entry. */
> +        wrmsrl(MSR_IA32_SYSENTER_ESP, stack_bottom);
> +        wrmsrl(MSR_IA32_SYSENTER_EIP, (unsigned long)sysenter_entry);
> +        wrmsr(MSR_IA32_SYSENTER_CS, __HYPERVISOR_CS, 0);
> +    }
> +
> +    /* Trampoline for SYSCALL entry from compatibility mode. */
> +    wrmsrl(MSR_CSTAR, stub_va);
> +    offset += write_stub_trampoline(stub_page + (stub_va & ~PAGE_MASK),
> +                                    stub_va, stack_bottom,
> +                                    (unsigned long)cstar_enter);
> +
> +    /* Don't consume more than half of the stub space here. */
> +    ASSERT(offset <= STUB_BUF_SIZE / 2);
> +
> +    unmap_domain_page(stub_page);
> +
> +    /* Common SYSCALL parameters. */
> +    wrmsrl(MSR_STAR, XEN_MSR_STAR);
> +    wrmsrl(MSR_SYSCALL_MASK, XEN_SYSCALL_MASK);
> +}

These want adjusting to use wrmsrns(), similarly to the previous patch. 
Fixed locally.

~Andrew